U.S. patent application number 11/773525 was filed with the patent office on 2009-01-08 for temperature compensation for pneumatic pumping system.
This patent application is currently assigned to BAXTER INTERNATIONAL INC.. Invention is credited to Gideon Hecht, Bruce E. Huitt, Douglas E. Vincent.
Application Number | 20090012447 11/773525 |
Document ID | / |
Family ID | 40222033 |
Filed Date | 2009-01-08 |
United States Patent
Application |
20090012447 |
Kind Code |
A1 |
Huitt; Bruce E. ; et
al. |
January 8, 2009 |
TEMPERATURE COMPENSATION FOR PNEUMATIC PUMPING SYSTEM
Abstract
Temperature compensation is applied to correct for temperature
mismatch between a reference chamber and a disposable chamber in a
pneumatic pumping system for dialysis fluid for peritoneal
dialysis. The mismatch creates an error in the calculation of
pumping volume of dialysate fluid. Applying a correction for the
temperature mismatch helps to more precisely control the volume of
dialysate that is metered to the patient. Also disclosed are ways
to keep temperatures constant and to use temperature sensors to
accurately measure the temperatures of the chambers. In other
aspects, the temperature of the dialysate fluid itself may be
measured and used to apply a correction to the volume of fluid that
is pumped to the patient.
Inventors: |
Huitt; Bruce E.; (Odessa,
FL) ; Vincent; Douglas E.; (Pelham, NH) ;
Hecht; Gideon; (Seminole, FL) |
Correspondence
Address: |
BAXTER HEALTHCARE CORPORATION
1 BAXTER PARKWAY, DF2-2E
DEERFIELD
IL
60015
US
|
Assignee: |
BAXTER INTERNATIONAL INC.
Deerfield
IL
BAXTER HEALTHCARE S.A.
Wallisellen
|
Family ID: |
40222033 |
Appl. No.: |
11/773525 |
Filed: |
July 5, 2007 |
Current U.S.
Class: |
604/28 |
Current CPC
Class: |
G16H 20/40 20180101;
A61M 2205/3372 20130101; G06F 17/40 20130101; G01F 15/02 20130101;
A61M 2205/12 20130101; A61M 1/28 20130101; A61M 2205/3396 20130101;
G06F 19/00 20130101; A61M 1/166 20140204 |
Class at
Publication: |
604/28 |
International
Class: |
A61M 1/14 20060101
A61M001/14 |
Claims
1. A method of compensating for temperature error in a pneumatic
pumping system of a peritoneal dialysis system, the peritoneal
dialysis system including a pumping mechanism with a disposable
cassette and at least one disposable chamber, wherein fluid
communication is established between the peritoneal dialysis system
and a peritoneal cavity of a patient, the method comprising:
calculating a corrected volume of air in a disposable chamber, V d
= V r * ( Pr 2 - Pr 1 ) ( Pd 1 - Pd 2 ) ##EQU00006## wherein
V.sub.d is the corrected volume, V.sub.r is a reference volume of
the chamber, and (Pr.sub.2-Pr.sub.1) is a pressure difference
between a high pressure in the reference chamber and a pressure in
the reference chamber after equalization, and (Pd.sub.1-Pd.sub.2)
is a pressure difference between a high pressure in the disposable
chamber and a pressure in the delivery chamber after equalization,
wherein pressures after equalization comprise pressure measurements
after the delivery and reference chambers are connected;
multiplying the result by a ratio of a temperature in the
disposable chamber to a temperature in the reference chamber
T.sub.d/T.sub.r; and multiplying the result by a ratio of
Tr.sub.cal/Td.sub.cal, wherein Tr.sub.cal is a temperature in the
reference chamber during a calibration procedure and Td.sub.cal is
a temperature in the disposable chamber during the calibration
procedure, wherein the temperature during a calibration procedure
is constant in the reference and disposable volumes; calculating a
corrected volume of air delivered by the disposable chamber of the
system by applying the correction factor; and correcting a volume
of dialysate pumped by the dialysis system using the corrected
volume of air.
2. The method of claim 1, further comprising storing Tr.sub.cal,
Td.sub.cal, or the ratio of Tr.sub.cal/Td.sub.cal in a memory of
the dialysis system.
3. The method of claim 1, further comprising taking the temperature
of the disposable chamber by a sensor located within the disposable
chamber or in physical contact with a wall of the disposable
chamber.
4. The method of claim 1, wherein the pneumatic pumping system
comprises two disposable chambers, and the temperature of the
disposable chamber is taken by one sensor located in one of the
disposable chambers or by a sensor located in each of the
disposable chambers.
5. The method of claim 1, wherein the temperature of the reference
chamber is taken by a sensor located within the reference chamber
or in physical contact with a wall of the reference chamber.
6. The method of claim 1, wherein the pneumatic pumping system
comprises two reference chambers, and the temperature of the
reference chamber is taken by one sensor located in one of the
reference chambers or by a sensor located in each of the reference
chambers.
7. A method of compensating for temperature error in a dialysis
system, the dialysis system including a pumping mechanism with a
disposable cassette and at least two disposable chambers, wherein
fluid communication is established between the peritoneal dialysis
system and a peritoneal cavity of a patient, the method comprising:
measuring a temperature of a particular pumping chamber; selecting
an offset temperature, said offset temperature dependent on the
temperature and on the particular pumping chamber; applying the
offset temperature to the measured temperature of the particular
pumping chamber; and correcting a volume of air pumped by the
particular pumping chamber using the formula V d = V r * T do * Tr
cal * ( Pr 2 - Pr 1 ) T r * Td cal * ( Pd 1 - Pd 2 ) ##EQU00007##
wherein V.sub.d is the corrected volume, V.sub.r is the nominal
chamber volume, T.sub.do is the temperature of the chamber with the
offset applied, T.sub.r is the temperature of the reference chamber
of the dialysis system, Tr.sub.cal/Td.sub.cal is a factory
correction factor calibrated at 37 C, and
(Pr.sub.2-Pr.sub.1)/(Pd.sub.1-Pd.sub.2) is a ratio of the pressure
differences of the reference chamber and the pumping chamber before
and after equalization.
8. The method of claim 7, wherein the temperature of the particular
pumping chamber is measured by a sensor in an air path or attached
to a wall inside the chamber.
9. The method of claim 7, wherein a table of offset temperatures is
stored in a memory of the dialysis system.
10. The method of claim 7, further comprising calculating the
offset temperatures by measuring a thermal time constant for the
particular chamber and applying the thermal time constant in a
correction factor to compensate for the temperature of the chamber
and a temperature of the air pumped.
11. The method of claim 10, further comprising storing the
correction factors and the thermal time constant in a memory of the
dialysis system.
12. A method of compensating for temperature error in a pneumatic
pumping system of a peritoneal dialysis system, the peritoneal
dialysis system including a pumping mechanism with a membrane and a
disposable cassette and at least one disposable chamber, wherein
fluid communication is established between the peritoneal dialysis
system and a peritoneal cavity of a patient, the method comprising:
calculating a corrected volume of air in a disposable chamber, V d
= V r * ( Pr 2 - Pr 1 ) ( Pd 1 - Pd 2 ) ##EQU00008## wherein
V.sub.d is the corrected volume, V.sub.r is a reference volume of
the chamber, and (Pr.sub.2-Pr.sub.1) is a pressure difference
between a high pressure in the reference chamber and a pressure in
the reference chamber after equalization, and (Pd.sub.1-Pd.sub.2)
is a pressure difference between a high pressure in the disposable
chamber and a pressure in the delivery chamber after equalization,
wherein pressures after equalization comprise pressure measurements
after the delivery and reference chambers are connected;
multiplying the result by a ratio of a temperature of a membrane
adjacent the disposable chamber to a temperature in the reference
chamber, T.sub.m/T.sub.r; and multiplying the result by a ratio of
Tr.sub.cal/Td.sub.cal, wherein Tr.sub.cal is a temperature in the
reference chamber during a calibration procedure and Td.sub.cal is
a temperature in the disposable chamber during the calibration
procedure, wherein the temperature during a calibration procedure
is constant in the reference and disposable volumes; calculating a
corrected volume of air delivered by the disposable chamber of the
system by applying the correction factor; and correcting a volume
of dialysate pumped by the dialysis system using the corrected
volume of air.
13. The method of claim 12, further comprising a step of estimating
a temperature of air pumped by the pneumatic pumping system and
using the estimate to correct the volume of air pumped.
14. The method of claim 12, further comprising a step of sensing a
temperature of air pumped by the pneumatic pumping system and using
the estimate to correct the volume of air pumped.
15. A method of compensating for temperature error in a pneumatic
pumping system of a peritoneal dialysis system, the peritoneal
dialysis system including a pumping mechanism with a membrane and a
disposable cassette and at least one disposable chamber, wherein
fluid communication is established between the peritoneal dialysis
system and a peritoneal cavity of a patient, the method comprising:
measuring a temperature of air pumped from a disposable chamber;
calculating a corrected volume of air in the disposable chamber, V
d = V r * ( Pr 2 - Pr 1 ) ( Pd 1 - Pd 2 ) ##EQU00009## wherein
V.sub.d is the corrected volume, V.sub.r is a reference volume of
the chamber, and (Pr.sub.2-Pr.sub.1) is a pressure difference
between a high pressure in the reference chamber and a pressure in
the reference chamber after equalization, and (Pd.sub.1-Pd.sub.2)
is a pressure difference between a high pressure in the disposable
chamber and a pressure in the delivery chamber after equalization,
wherein pressures after equalization comprise pressure measurements
after the delivery and reference chambers are connected;
multiplying the result by a ratio of an offset temperature to a
temperature in the reference chamber, T.sub.m/T.sub.r, the offset
temperature comprising the measured temperature of the air
corrected by an offset and wherein the offset depends on the
measured temperature and the disposable chamber; and multiplying
the result by a ratio of Tr.sub.cal/Td.sub.cal, wherein Tr.sub.cal
is a temperature in the reference chamber during a calibration
procedure and Td.sub.cal is a temperature in the disposable chamber
during the calibration procedure, wherein the temperature during a
calibration procedure is constant in the reference and disposable
volumes; calculating a corrected volume of air delivered by the
disposable chamber of the system by applying the correction factor;
and correcting a volume of dialysate pumped by the dialysis system
using the corrected volume of air.
16. The method of claim 15, wherein a table of offset temperatures
is stored in a memory of the dialysis system.
17. The method of claim 15, further comprising calculating the
offset temperature by measuring a thermal time constant for the
disposable chamber and applying the thermal time constant in a
correction factor to compensate for the temperature chamber and a
temperature of the air pumped.
18. A method of controlling pneumatic air temperature in a dialysis
system, the method comprising: mounting a first strip heater in a
pneumatic distribution manifold, the pneumatic distribution
manifold further comprising a precision reference volume;
controlling a temperature of the first strip heater by monitoring a
temperature of the manifold; sensing a temperature of air in a
disposable chamber; calculating a corrected volume of air delivered
by the disposable chamber and correcting a volume of dialysate
pumped by the dialysis system using the corrected volume of
air.
19. The method of claim 18, further comprising: mounting a positive
pressure air tank and a negative pressure air tank within an
enclosure, which may be the pneumatic distribution manifold or may
be a different enclosure; providing a second electrical strip
heater within the enclosure; and heating the enclosure and
controlling a temperature of the tanks within the enclosure with
the second electrical strip heater.
20. The method of claim 18, wherein the first electrical strip
heater is controlled by at least one of a thermocouple and a
thermistor.
21. The method of claim 20, wherein a set point of at least one of
the first and second electrical strip heaters is set above a
desired temperature of dialysate fluid to be delivered by the
dialysis system.
22. The method of claim 18, wherein the dialysis system comprises
two delivery chambers and the volume of air delivered is corrected
using an average of a temperature of the two chambers.
23. The method of claim 18, wherein the dialysis system comprises
two delivery chambers and the volume of air delivered is corrected
using an average of a temperature of the two chambers, and wherein
a correction applied comprises a correction factor of a ratio of a
temperature of a disposable to a temperature of one or more
reference chambers, and optionally, a second correction applied
comprises a ratio of a calibration temperature of a reference
chamber to a calibration temperature of the disposable.
24. The method of claim 18, wherein the pneumatic distribution
manifold is contained within a door assembly of the dialysis
system.
25. The method of claim 19, wherein the pneumatic distribution
manifold, the enclosure, the air tanks, and the disposable chamber
are contained within a door assembly of the dialysis system.
Description
BACKGROUND
[0001] The field is that of medical instruments, pneumatic pumping
of small, precise volumes in medical instruments, and temperature
compensation for medical instruments and precise fluid
movements.
[0002] Pneumatic pumping systems are used to pump many fluids,
including fluids with medical applications. One such application is
a fluid management system used to pump dialysate fluid for an
automated peritoneal dialysis system. In general terms, persons
whose kidneys do not function adequately may use the peritoneal
membrane instead. Diffusion and osmotic exchanges take place across
the peritoneal membrane between the dialysis solution and the
bloodstream. These exchanges remove the waste products normally
excreted by the kidneys. The waste products typically include
sodium and chloride ions, and other compounds normally excreted
through the kidneys, such as urea, creatinine, and water.
[0003] In a typical treatment, dialysis fluid enters the patient,
dwells for a period of time, usually several hours, and is then
removed. The cycle may be repeated as often as the attending
physician believes is necessary. Peritoneal Dialysis (PD) may be
used in the form of continuous ambulatory peritoneal dialysis
(CAPD), in which a patient performs a PD cycle about four times a
day, using a cycle of infusing the peritoneal dialysis fluid, dwell
for a period of time, and then draining the PD fluid. Each cycle
lasts for several hours. In automated peritoneal dialysis (APD), a
cycler performs the drain, fill and dwell tasks while the patient
sleeps at night. There are also a number of other recognized
treatment regimes, such as intermittent PD ("IPD") or continuous
cycling PD ("CCPD"), in which the cycle is performed about 6 times
per day and the peritoneal cavity is completely drained during the
drain portions of the cycle.
[0004] Regardless of the particular regimen used, peritoneal
dialysis is a difficult procedure for patients. In order to
preserve patient comfort, and thus to encourage the patient to
return for treatment, it is important to accurately control the
volume of dialysate that is pumped into the patient. The volume of
dialysis fluid is important for several reasons. Excess volume by
itself could cause patient discomfort or injury, while too little
volume will result in a longer and less-effective change therapy.
The difference in temperature between the pneumatic pumping system
at ambient temperature and the temperature at which it was
calibrated (at the factory) detracts from the accuracy of the
volume of the dialysate pumped to the patient. What is needed is a
better way to compensate for the difference in these
temperatures.
SUMMARY
[0005] A first embodiment is a method of compensating for
temperature error in a pneumatic pumping system of a peritoneal
dialysis system, the peritoneal dialysis system including a pumping
mechanism with a disposable cassette and at least one disposable
chamber, wherein fluid communication is established between the
peritoneal dialysis system and a peritoneal cavity of a patient.
The method includes calculating a corrected volume of air in a
disposable chamber,
V d - V t * ( Pr 2 - Pr 1 ) ( Pd 1 - Pd 2 ) ##EQU00001##
wherein V.sub.d is the corrected volume, V.sub.r is a reference
volume of the chamber, (Pr.sub.2-Pr.sub.1) is a pressure difference
between a high pressure in the reference chamber and a pressure in
the reference chamber after equalization, (Pd.sub.1-Pd.sub.2) is a
pressure difference between a high pressure in the disposable
chamber and a pressure in the delivery chamber after equalization,
wherein pressures after equalization comprise pressure measurements
after the delivery and reference chambers are connected. The method
also includes multiplying the result by a ratio of a temperature in
the disposable chamber to a temperature in the reference chamber
T.sub.d/T.sub.r, and multiplying the result by a ratio of
Tr.sub.cal/Td.sub.cal, wherein Tr.sub.cal is a temperature in the
reference chamber during a calibration procedure and Td.sub.cal is
a temperature in the disposable chamber during the calibration
procedure, wherein the temperature during a calibration procedure
is constant in the reference and disposable volumes. The method
then includes calculating a corrected volume of air delivered by
the disposable chamber of the system by applying the correction
factor, and correcting a volume of dialysate pumped by the dialysis
system using the corrected volume of air.
[0006] Another embodiment is a method of compensating for
temperature error in a dialysis system, the dialysis system
including a pumping mechanism with a disposable cassette and at
least two disposable chambers, wherein fluid communication is
established between the peritoneal dialysis system and a peritoneal
cavity of a patient. The method includes measuring a temperature of
a particular pumping chamber, selecting an offset temperature, said
offset temperature dependent on the temperature and on the
particular pumping chamber, applying the offset temperature to the
measured temperature of the particular pumping chamber, and
correcting a volume of air pumped by the particular pumping chamber
using the formula
V d = V r * T d 0 * Tr cal * ( Pr 2 - Pr 1 ) T r * Td cal * ( Pd 1
- Pd 2 ) ##EQU00002##
wherein V.sub.d is the corrected volume, V.sub.r is the nominal
chamber volume, T.sub.do is the temperature of the chamber with the
offset applied, T.sub.r is the temperature of the reference chamber
of the dialysis system, Tr.sub.cal/Td.sub.cal is a factory
correction factor calibrated at 37 C, and
(Pr.sub.2-Pr.sub.1)/(Pd.sub.1-Pd.sub.2) is a ratio of the pressure
differences of the reference chamber and a pumping chamber before
and after equalization.
[0007] Another embodiment is a method of compensating for
temperature error in a pneumatic pumping system of a peritoneal
dialysis system, the peritoneal dialysis system including a pumping
mechanism with a membrane and a disposable cassette and at least
one disposable chamber, wherein fluid communication is established
between the peritoneal dialysis system and a peritoneal cavity of a
patient. The method includes calculating a corrected volume of air
in a disposable chamber,
V d - V r * ( Pr 2 - Pr 1 ) ( Pd 1 - Pd 2 ) ##EQU00003##
wherein V.sub.d is the corrected volume, V.sub.r is a reference
volume of the chamber, and (Pr.sub.2-Pr.sub.1) is a pressure
difference between a high pressure in the reference chamber and a
pressure in the reference chamber after equalization, and
(Pd.sub.1-Pd.sub.2) is a pressure difference between a high
pressure in the disposable chamber and a pressure in the delivery
chamber after equalization, wherein pressures after equalization
comprise pressure measurements after the delivery and reference
chambers are connected. The method also includes multiplying the
result by a ratio of a temperature of a membrane adjacent the
disposable chamber to a temperature in the reference chamber,
T.sub.m/T.sub.r, multiplying the result by a ratio of
Tr.sub.cal/Td.sub.cal, wherein Tr.sub.cal is a temperature in the
reference chamber during a calibration procedure and Td.sub.cal is
a temperature in the disposable chamber during the calibration
procedure, wherein the temperature during a calibration procedure
is constant in the reference and disposable volumes. The method
also includes calculating a corrected volume of air delivered by
the disposable chamber of the system by applying the correction
factor, and correcting a volume of dialysate pumped by the dialysis
system using the corrected volume of air.
[0008] Another embodiment is method of compensating for temperature
error in a pneumatic pumping system of a peritoneal dialysis
system, the peritoneal dialysis system including a pumping
mechanism with a membrane and a disposable cassette and at least
one disposable chamber, wherein fluid communication is established
between the peritoneal dialysis system and a peritoneal cavity of a
patient. The method includes measuring a temperature of air pumped
from a disposable chamber, and calculating a corrected volume of
air in the disposable chamber,
V d = V r * ( Pr 2 - Pr 1 ) ( Pd 1 - Pd 2 ) ##EQU00004##
wherein V.sub.d is the corrected volume, V.sub.r is a reference
volume of the chamber, and (Pr.sub.2-Pr.sub.1) is a pressure
difference between a high pressure in the reference chamber and a
pressure in the reference chamber after equalization, and
(Pd.sub.1-Pd.sub.2) is a pressure difference between a high
pressure in the disposable chamber and a pressure in the delivery
chamber after equalization, wherein pressures after equalization
comprise pressure measurements after the delivery and reference
chambers are connected. The method also includes multiplying the
result by a ratio of an offset temperature to a temperature in the
reference chamber, T.sub.m/T.sub.r, the offset temperature
comprising the measured temperature of the air corrected by an
offset and wherein the offset depends on the measured temperature
and the disposable chamber, and multiplying the result by a ratio
of Tr.sub.cal/Td.sub.cal, wherein Tr.sub.cal is a temperature in
the reference chamber during a calibration procedure and Td.sub.cal
is a temperature in the disposable chamber during the calibration
procedure, wherein the temperature during a calibration procedure
is constant in the reference and disposable volumes. The method
then includes calculating a corrected volume of air delivered by
the disposable chamber of the system by applying the correction
factor and then correcting a volume of dialysate pumped by the
dialysis system using the corrected volume of air.
[0009] Another embodiment is a method of controlling pneumatic air
temperature in a dialysis system. The method includes mounting a
first strip heater in a pneumatic distribution manifold, the
pneumatic distribution manifold further comprising a precision
reference volume, controlling a temperature of the first strip
heater by monitoring a temperature of the manifold, sensing a
temperature of air in a disposable chamber, calculating a corrected
volume of air delivered by the disposable chamber, and correcting a
volume of dialysate pumped by the dialysis system using the
corrected volume of air.
[0010] Additional embodiments, features and advantages are
described herein, and will be apparent from, the following Detailed
Description and the figures.
BRIEF DESCRIPTION OF THE FIGURES
[0011] FIGS. 1A, 1B and 1C depict systems for peritoneal
dialysis;
[0012] FIG. 2 depicts a pumping system for dialysis fluid;
[0013] FIG. 3 depicts a system for an actively controlled pneumatic
path with temperature compensation;
[0014] FIG. 4 depicts a system for a minimally active controlled
pneumatic path;
[0015] FIG. 5 depicts temperature error in prior art pneumatic
systems;
[0016] FIG. 6 depicts temperature error in fluid pumped to a
patient;
[0017] FIG. 7 depicts temperature error in fluid pumped from a
patient;
[0018] FIG. 8 depicts time lag between start-up and the time the
piston achieves thermal steady-state or equilibrium
temperature;
[0019] FIG. 9 depicts the time lag and temperature differences
between the dialysate fluid and the pistons;
[0020] FIG. 10 depicts the time lag and temperature differences
along with an estimate of the temperature of one of the
pistons;
[0021] FIG. 11 depicts the volumetric error based on a first method
of applying a steady-state temperature offset for a pneumatic
pumping system; and
[0022] FIG. 12 depicts the volumetric error based on a second
method of applying a steady-state temperature offset for a
pneumatic pumping system.
DETAILED DESCRIPTION
[0023] In pneumatic pumping systems, a given volume of air from a
pumping or dispensing chamber is used to displace a volume of
liquid (dialysate) that is dispensed to a patient. In general
terms, the volume of liquid dialysate dispensed to the patient is
controlled by controlling the amount of air dispensed from the
pumping chamber or chambers. The amount of air is calculated using
the well known gas law, and then corrected for the temperature and
pressure of the air. The "volume" of air provided by the pumping
chamber is clearly a misnomer, since the known volume of a pumping
chamber can hold vastly differing quantities of air, depending on
the temperature and the pressure of the air. Assuming that air is a
perfect gas precisely obeying the gas law, what is really sought is
the number of gas molecules in the volume, corrected for
temperature and pressure. Nevertheless, the volume of the chamber
is relatively constant, within the limits of ambient temperature
excursions and the thermal expansion of the material used for the
chamber walls. We can therefore use chamber volume or gas volume as
a substitute for the calculating the number of molecules and then
converting the number of molecules to a volume on the dialysis
machine or disposable, that volume corresponding conveniently to
the volume of dialysate. That substitution is well known to those
having skill in the art, and that substitution is used in this
patent for temperature compensation in a pneumatic pumping system.
In addition, all temperatures used for temperature correction
factors refer to absolute temperatures, e.g., in degrees Kelvin or
Rankine.
[0024] In one embodiment, a testing mechanism for testing a medical
fluid is incorporated with a medication delivery system such as a
dialysis machine 10, illustrated in FIG. 1. In another embodiment,
a testing mechanism for testing a medical fluid is incorporated
within a different medication delivery system such as an infusion
pump (not shown). The testing mechanisms of the present disclosure
are illustrated and described as cooperating with a dialysis
machine, but it should be understood by one skilled in the art that
the dialysis machine is an exemplary medication delivery system and
the testing mechanism can be used in cooperation with any
medication delivery system that delivers a medical fluid to a
patient. It should be understood by one skilled in the art that the
testing mechanisms of the present disclosure can be incorporated
within a medication delivery system, or the testing mechanisms of
the present disclosure can be formed as devices separate from the
medication delivery system, but which maintain an operative
connection to the medication delivery system. A medical delivery
system for a particular medical fluid may include a bag or
container in which the medical fluid is typically or commercially
furnished, and may include integral or appended tubing for delivery
to the patient.
[0025] A plurality of medication containers 12 containing medical
fluid are attached to the dialysis machine 10, as shown in FIG. 1.
In an embodiment, each medication container 12 contains the same
medical fluid. For example, each medication container 12 contains a
dialysis solution, or dialysate, containing glucose molecules. In
another embodiment, each medication container 12 includes different
medical fluids that are combinable within the dialysis machine 10
prior to being delivered to a patient. A disposable line set 14
operatively connects each medication container 12 to the dialysis
machine 10. A line 16 leading to the patient, formed of a
disposable tube, delivers the medical fluid from the dialysis
machine 10 to the patient (not shown). A drain line 18 formed of a
disposable tube directs spent dialysate and blood waste products
from the dialysis machine 10 to a drain container (not shown).
[0026] The dialysis machine 10 receives a disposable fluid
containment device, such as a cassette 20 or a portion of the line
set 14, to which the medication containers 12 are connected, as
illustrated in FIG. 1B. The dialysis machine 10 includes a recessed
area 22 for receiving the disposable cassette 20. The recessed area
22 is sized and shaped to provide a snug fit with the disposable
cassette 20 to ensure the cassette 20 is properly aligned within
the recessed area 22. In particular, the opposing side walls 24 of
the cassette 20 fit snuggly within opposing inner walls 26 of the
recessed area 22. As shown in FIG. 1C, the cassette 20 includes a
pair of flexible membranes 28 attached to opposing surfaces of a
rigid structure 30. The flexible membranes 28 are operatively
attached to the rigid structure 30 to provide, for example, pumping
movement to allow the medical fluid received from the medication
containers 12 and delivered to the patient.
[0027] Pumping of dialysate fluid can be described with respect to
the prior art fluid pumping system 31 depicted in FIG. 2. A
preferably two-headed pump or compressor (not shown) is connected
with a series of valves 33 to a reference volume chamber 32, which
is also connected via valve 35 to the atmosphere and to a reference
pressure gauge Pr. Using one head, the pump provides a source of
pressurized air, typically about 4-5 psig, to a positive pressure
tank 37. Using the vacuum head, the pump can also act as a source
of air at a lower pressure for the "negative" pressure tank 36. The
"negative" pressure tank typically has a pressure less than
atmospheric by about 4-5 psi, that is, at about 9-10 psia. In order
to pump dialysate fluid, the pumping chamber 38, with membrane 34,
which is equipped with a pressure gauge Pd, is filled with an
unknown amount of dialysate fluid from a fluid reservoir 39. An
amount of air is measured in the reference volume chamber 33 and is
then pumped into the pumping chamber 38, with the appropriate
valves opened or closed as is necessary.
[0028] Thermal Compensation
[0029] Air at one temperature flows from the fixed air volume 33
into the air or right side of disposable chamber 38, which may be
at a second temperature. When the air flows in, dialysate on the
opposite side of the membrane 34 may be at yet another temperature,
and the dialysate is pushed to the fluid outlet, i.e., to tubing
that leads to the patient. After this cycle, the pumping or
dispensing chamber is then filled as fluid from the fluid inlet
flows into the pumping or dispensing chamber. The fluid may be at
yet another temperature. The thermal energy flow through the
membrane causes a temperature difference between pumping chamber 38
and reference chamber 32.
[0030] There are many ways to compensate for the temperature
differences in these systems. One way used to estimate the volume
of air in the dispensing chamber, V.sub.d, is to use simple
pressure correction of the form
V.sub.d=V.sub.r(Pr.sub.2-Pr.sub.1)/(Pd.sub.1-Pd.sub.2) (Equation
1)
[0031] where Pr.sub.1 and Pr.sub.2 are the pressures measured in
the reference chamber at atmospheric pressure and after
equalization respectively with the pressurized dispensing chamber
at a high pressure. Pd.sub.1 and Pd.sub.2 are the pressures
measured in the dispensing chamber at high pressure and after the
two chambers are equalized. V.sub.r is the reference chamber volume
as calibrated at the factory. The calculation provides a measure of
the air volume in the dispensing chamber. Calculating the volume
differences in air volume of the dispensing chamber prior to and
after the movement of fluid allows the calculation of the displaced
fluid. As noted, however, this calculation does not account for the
error caused by temperature difference.
[0032] One improvement therefore uses measurement of temperature in
both the dispensing chamber, also known as the piston area, and the
volume reference chamber to make a correction to the air volume
calculation. A thermocouple or thermistor is preferably placed in
an area of the dispensing chamber or piston where it will readily
detect the temperature of the air. In one embodiment, the
temperature-measuring device is placed within the chamber, in
physical contact with the walls of the piston chamber, which is
filled with a foamed material to minimize thermodynamic effects.
This contact enables the temperature measuring device to more
accurately measure the air temperature, because the air quickly
equilibrates with its surroundings, the material in the dispensing
chamber or piston, due to its low thermal mass. In the same manner,
a thermocouple may be placed in the reference chamber to complete
the data necessary for a more accurate calculation. The
thermocouple in the reference chamber is preferably also placed in
contact with the walls of the chamber.
[0033] Using these temperatures, a correction factor of the
temperature of the dispensing chamber or piston, T.sub.d, divided
by the temperature of the reference chamber, T.sub.r, may be used.
In addition, it has been discovered that the correction is improved
by using an additional correction factor, when pneumatic piston air
temperatures are not at the temperatures used to calibrate the
system. The calibration is preferably accomplished at a
manufacturing facility or other convenient facility, and is
accomplished using fluids at body temperature, i.e., 37 C. The
additional factor is the temperature in the reference chamber
divided by the temperature in the dispensing chamber or piston,
i.e., Tr.sub.cal/Td.sub.cal. This correction factor may be stored
in a memory of the dialysis machine controller or in the dialysis
controller. In most instances, the air used to pump fluid will be
cooler than body temperature, and therefore will have a lower
volume than air (and dialysate fluid) at body temperature. In most
instances, then, the error will be negative, i.e., a lower amount
of fluid will be pumped than is needed by the patient.
[0034] The correction factor is thus
Vd = Vr * T d * Tr cal * ( Pr 2 - Pr 1 ) T r * Td cal * ( Pd 1 - Pd
2 ) ( Equation 2 ) ##EQU00005##
[0035] This allows a user to quickly calibrate a particular piston
or dispensing chamber using factory calibration data, and
temperature and pressure data available in the unit under
discussion. This correction may be applied to a variety of
mechanical configurations, a few of which may also be described
below. In other embodiments, the temperature of the disposable
T.sub.d may be estimated using the membrane temperature,
T.sub.m.
[0036] The corrected volume of air pumped by the disposable may
then be used to pump the correct amount of dialysate. In one
example, the expected value of a single stroke of a disposable
chamber is 20.0 ml, and the actual volume is 20.8. A normal
dialysis session, delivering 2000 ml, would require 100 pumping
chamber strokes. In this instance, each stroke would deliver 0.8 ml
extra, and fewer strokes may be required to deliver 2000 ml. The
dialysis machine may be programmed to keep track of the volume,
which for the sake of simplicity, is assumed to be constant on each
stroke. As the session nears its end, say 95 strokes, a total of
about 1976 ml will have been delivered. Thus, only 1 additional
full stroke will bring the total to about 1997 ml. The disposable
may then use mini or partial strokes to bring the total to 2000 ml
or may be programmed to stop when the total volume is within, say,
0.5% or 1%. In this instance, the correction avoids 4 extra stokes,
about 83 ml, a 4% error.
[0037] Temperature Control
[0038] Even with good temperature compensation, as described above,
it would be desirable, and the patient may prefer, that an active
form of temperature control should be exercised. For example,
gentle warming of the dialysate fluid to body temperature, along
with all other components of the system, would minimize the need
for temperature compensation, because the fluid and all measuring
and dispensing tools would be at the same (body) temperature. In
another aspect, the time required for the pumping and dispensing
unit to reach thermal equilibrium is reduced and the thermal
mismatch between the components and body temperature is minimized.
The temperature compensation discussed above may still be applied,
but overall patient comfort is increased, as is the accuracy of the
fluid measurement. In still another aspect, a direct reading of the
dialysate fluid temperature may be taken with a thermocouple,
thermistor, or other temperature sensor or temperature element
placed into the dialysate fluid itself rather than into the
pneumatic paths.
[0039] FIG. 3 discloses a dialysate fluid dispensing system with
temperature control, with which temperature compensation may also
be used. The fluid pumping system 40 includes an enclosed manifold
41 with a precision reference volume 42 and a strip heater 43 for
maintaining a desired temperature. The system also includes a heat
controlled enclosure 44 for the positive and negative tanks 44h,
441, and a door assembly 47 with integral dispensing chambers or
pistons 47l and 47r. A number of temperature sensors, as will be
explained, are used throughout the system. In addition, insulation
48 is preferably used for the tubing that connects the various
parts of the system. The insulation need not be complicated, and
may be simple runs of foam between the enclosures. As will be
recognized by those having ordinary skill in this art, all of these
components may reside in a peritoneal dialysis system, which will
include a housing for most of the above components, including a
door for the disposables and a warming tray for a bag of dialysate
fluid. It is preferable if the fluid pumping system components as
depicted in FIG. 3 are made part of an integrated door assembly.
This places many parts close together and for all practical
purpose, integrates them into the door assembly. Many of these
components are disclosed and explained in U.S. Pat. No. 5,474,683,
which is hereby incorporated by reference, as though each page were
explicitly set forth in this patent.
[0040] Enclosed manifold 41 may be enclosed within the system
housing by thin layers of foam or other insulation. The manifold
encloses a number of components which are not shown. These
typically include a manifold assembly, a top plate, bottom plate,
pressure bladder, occluder bladder, and a number of in and out
pneumatic valves. While temperature compensation is important, it
is recognized that the temperature differential in most instances
will be limited to a difference from room temperature, 25 C
(77.degree. F.), to body temperature, about 37 C (98.6.degree. F.),
for a twelve-degree C. temperature gradient. From the perspective
of the patient receiving cool fluid into a bloodstream, this is a
significant difference. Even in an extreme situation, perhaps 20 C
(68.degree. F.), the gradient would be at most 17 C (about
30.degree. F.) temperature difference. From a heat-transfer
perspective, however, the temperature difference, or driving force
for heat transfer, is relatively small. The manifold enclosure
provides a relatively uniform temperature. The manifold enclosure
includes a reference chamber 42, which includes separate left and
right reference volumes, 42l, 42r, and also includes at least part
of the tubing that connects the reference chamber to positive and
negative tanks 44h, 44l, and to the pistons 47r, 47l. Chamber 42
may be an aluminum block, or may be made of another material.
Alternately, the chambers may be separate pieces. Chamber 41 may
also house valves A, P, N, D for operating the pneumatic
system.
[0041] The reference chamber and the tubing (pneumatic pathways)
are heated by one or more heaters, such as electric tape strip
heaters 43. The heaters may be applied directly to the surfaces of
the reference chamber and the tubing, or may be placed elsewhere
with the manifold. The heaters are preferably controlled by at
least one controller 43a using a PID control scheme. Other control
schemes, such as using pulse-width-modulation may be used instead.
At least one temperature sensor, T1 should be applied to each
reference chamber, and preferably another, T2, at another point
within the enclosure, to provide feedback to the controller. The
temperature of the reference volumes, 42l, 42r, may be used as
T.sub.r in the temperature compensation method described above.
[0042] As is well known, the positive and negative tanks 44h, 441,
are typically placed close to the reference chamber(s). In this
embodiment, the tanks are enclosed with a heat-controlled enclosure
44, the tanks in contact with a heater 45. Heater 45 may be a
heater rod, a strip heater, or other heater sufficient for mild
warming of the tanks. Enclosure 44 may include strips of insulating
plastic or foam, and should include at least one temperature sensor
T2 to provide feedback to a temperature controller 44a for
controlling the temperature of enclosure 44. Enclosure 44 will also
enclose some of the tubing that connects tanks 44h, 441 to the
valves within enclosed manifold 41. The valves for tanks 44h, 44l
may also be enclosed within enclosure 44.
[0043] The door assembly 47 housing the dispensing chambers or
pistons 47l, 47r. Each piston is preferably equipped with a
temperature sensor T1, the temperature sensors each sending a
signal back to a controller for the pneumatic pumping system. This
temperature, Td, may be used in the temperature compensation method
described above, and may also be displayed, for instance on a
digital readout of the controller. It has been found that some
temperature sensors are preferred. In the discussions above, the
preferred temperature sensor T1 may be a type K thermocouple with a
very fast time constant, such as 0.005 seconds. Temperature sensor
T2 may be a slower-acting type K thermocouple. Other types of
thermocouples may be used, and thermistors or other temperature
sensors may be used. These are merely the preferred elements.
[0044] It will be recognized that the thermal insulation and
thermal control scheme of FIG. 3 is an excellent, though
complicated, embodiment. It may be possible to implement a lesser
degree of insulation and control, while retaining most of the
advantages of the system of FIG. 3. A less-complicated embodiment
is depicted in FIG. 4. In this embodiment of a temperature control
system 50 for a pneumatic fluid pumping system, the reference
chambers 52l, 52r are enclosed and insulated in a enclosed manifold
51, again with at least one temperature sensor T2, attached to an
aluminum block 52 that may include chambers 52l, 52r. The
temperature from this sensor may be used as T.sub.r. Only the
tubing 58 that directly connects the reference chambers to the
disposables 57l, 57r is insulated. The disposables or pistons 57l,
57r, remain within the door assembly 57, each preferably equipped
with a T1 sensor that may be used as T.sub.d, e.g., T.sub.d-1 or
T.sub.d-r, respectively for the left and right pistons. The
positive and negative tanks 54h, 54l may not be inside a
heat-controlled enclosure. It will be recognized, however, that
there is little wasted space within the housing that encloses the
elements of a pneumatic pumping system. The heat given off by a
positive tank from warmer air when it is filled may be absorbed by
the surroundings. If there is an appreciable cooling effect from
the negative tank, it will likely be minimal, and will be also
compensated for by its surroundings. Some of these negative effects
can be overcome by enclosing the components of system 50 into an
integrated door assembly.
[0045] Thermal Results
[0046] A number of experiments were conducted using the pneumatic
pumping systems described above. In these experiments, an aluminum
reference chamber was used, attached to a pneumatic pressure
distribution manifold with thermal-conducting grease. The air tanks
and tubing, including the tubing to the disposables (pistons) were
insulated. Fast thermocouples were mounted to the left and right
reference chambers and to the left and right pistons. The membrane
between the pistons and the diaphragm of the cassette was equipped
with a fast thermocouple, and the manifold itself with a slow
thermocouple. The manifold, reference chamber, air tanks, and tubes
were heated to abut 56C.
[0047] In one series of experiments, as shown in FIG. 5, precision
scale measurements of pumped fluid volume were compared to nominal
volumes with no temperature correction applied. This series of
experiments concerns dialysis fluid pumped to the patient. The left
vertical axis represents the actual temperature of the dialysis
fluid that was pumped, in degrees C., and the right vertical axis
represents the volumetric error. The continuous line, labeled fluid
temperature, is the actual temperature and the unitary dots
represent the corresponding error, with no temperature correction
(no TC). The horizontal axis represents the time over which the
experiments took place. One conclusion is that the error is
correlated to temperature, and the greater the temperature
difference (from body temperature, 37 C), the greater the
volumetric error. Both the volumetric and temperature data have
been averaged for the left and right sides of the disposable or
pistons. Each dot represents about 1000 ml of transferred
fluid.
[0048] Another series of experiments was conducted, using the
temperature correction discussed above and incorporated as Equation
2 above. These experiments are depicted in FIG. 6, using the
experimental setup and the same framework as that of FIG. 5. The
continuous line represents the actual fluid temperature, while the
unitary dots represent error, based on the fluid management system
(FMS) corrected error, based on the temperature of the right and
left disposables (TDispL& R). In this set of experiments, the
error is greatly reduced, with extremes of about positive 0.30%
when the dialysate temperature is cool (15 C) and about negative
0.60% when the dialysate is at 35 C. This contrasts with an error
of about 7% with no temperature compensation, as shown in FIG. 5.
Accordingly, the temperature compensation of Equation 2
significantly reduces volumetric error in dispensing dialysis fluid
to a patient.
[0049] Even greater error correction was achieved in measuring the
volumes of dialysate pumped from the patient, as shown in FIG. 7.
These experiments were conducted in the same manner as the above,
with the continuous line representing actual fluid temperature, and
the volumetric error represented by unitary dots. Errors in fluid
volume pumped are again very low, now from about positive 0.50% to
about negative 0.50%, with fluid temperatures ranging from 15 C to
about 35 C. Again, both the volumetric and temperature data have
been averaged for the left and right sides of the disposable or
pistons. Each dot represents about 1000 ml of transferred
fluid.
[0050] Direct Fluid Measurement
[0051] Another improvement is the elimination of one or more
temperature sensors within the dispensing chambers or pistons.
Instead of using sensors in these chambers, it is possible to mount
a temperature sensor, such as a thermocouple, directly in the
dialysate outlet line, such as the outlet of the left or right
dispensing chamber, or both. Using these temperatures, an estimate
of T.sub.d, for either the right or left piston, or both, may then
be made. Once the pneumatic pumping system is in a steady-state
operating condition, temperature correction is fairly
straight-forward, as noted above. However, until that state is
reached, relatively cooler fluid being pumped cools the membrane
and the areas in contact with the membrane. The temperature of the
pumped dialysate fluid will therefore lag the temperature of the
air used to pump the fluid. A time constant, or temperature lag,
can be established empirically and used to apply measured fluid
temperature correction to the piston or dispensing chamber
temperature. The pneumatic pumping system can then be programmed to
apply this correction or lag to correct the predicted piston
temperatures of each chamber in order to compensate for pneumatic
volume, as discussed above in the section on thermal
compensation.
[0052] An illustration of the thermal lag problem is depicted in
FIG. 8. The temperature of the piston or dispensing chamber at
start-up is Temp1, while the temperature which it will eventually
reach is Temp2. In most instances, Temp1 will be at a lower
temperature, while Temp2 will be higher, but the principle applies
to corrections in either direction. It will take a finite period of
time until the system comes to a steady state and the two
temperatures are equal, and the error in the interim is the
difference between the two temperatures. FIGS. 9 and 10 depict the
result of tests to determine thermal lag and the differences in
actual operating systems. FIG. 9 depicts the temperature in the
left and right dispensing chambers or pistons, T.sub.d-1 and
T.sub.d-r, as compared with the actual temperature of dialysate
fluid, fluid temperature. As the graph shows, the dialysate
temperature, fluid temperature, change leads the piston temperature
change by as much as 7 or 8 degrees C. for many minutes, even
hours. Each horizontal marker is 5000 seconds, the 500 units
indicated, multiplied by 10 sec. As can also be seen in FIG. 9,
there is an offset between the fluid temperature, the solid line,
and the temperature of the pistons does not reach the fluid
temperature.
[0053] In FIG. 10, an estimate is made of the temperature of the
combined left and right chambers, and is displayed on the graph as
the dashed line. This estimate may then be applied as a correction
factor, or offset, to the measured temperature of dialysate fluid,
and used in an estimate of the piston temperatures. The estimate
may be made by calculating an ultimate temperature difference,
taking the 63.2% value of the difference, and then adding that
value to the detected temperature. For instance, in FIG. 10,
T.sub.1 for T.sub.d-1 is taken early in the process, as 12.5, while
T.sub.2 for T.sub.d-r is 16.3. In both cases, an estimate for the
next temperature, T.sub.2, is taken by letting T.sub.2 equal the
actual measured temperature of the dialysate less an offset
temperature. The offset temperature is taken as (1-e.sup.t/.tau.)
multiplied by the difference between the first temperature and the
dialysate temperature, where t is the estimated time lag in seconds
and .tau. is a time constant that is derived from the calibration
value when the chambers are calibrated. In testing, the value of
.tau. was determined to be 67 seconds and 197 seconds in different
experiments for different disposable configurations. Table 1 with
the results is shown below. The temperature was measured precisely,
to a tenth of a degree, and the correlation for accuracy is within
about 1 degree. Thus, even though temperature T.sub.2 and
T.sub.offset should add up to the long-term equilibrium value, 24
C, there is a discrepancy of +0.2.degree. C. for T.sub.d-r and
+0.8.degree. C. for T.sub.d-1.
TABLE-US-00001 TABLE 1 Estimating Piston Temperature using
Disposable/Membrane/Foam and Thermal Time Constant Column 6 7 1 2 3
4 5 Tau T.sub.offset .degree. C. Chamber T.sub.1 T.sub.2 Delta T,
.degree. C. Delta C * 0.632 T.sub.1 + col. 4 (63.2%) (@ 24 C.)
T.sub.d-r 12.5 20.9 8.4 5.3 17.8 67 -3.3 T.sub.d-l 16.3 22.9 6.6
4.2 20.5 197 -1.9
[0054] The value of .tau. will depend on the particular design of
the pumping chamber, and may also vary with the particular ambient
temperature of the apparatus. Thus, the particular offset
temperature used in the calculation will also vary depending on the
pumping chamber itself, or at least its design, and the temperature
of the pumping chamber. As also noted above, there can be
significant differences between left and right chambers in the same
machine. This may be due to the environment within the disposable
or the dialysis machine, such as the precise location of sources of
heat and cold with respect to the left and right pumping chambers.
The difference may also be due to minute differences in the room in
which dialysis takes place, such as one side of the room being a
few degrees warmer or cooler. There may be a source of cooled or
heated air that impacts the dialysis machine, and the left and
right chambers, in different ways, if the left or right chamber is
closer to the source or heating or cooling, and thus one side is
more affected than the other.
[0055] Thus, the offset temperatures are calculated and applied to
the fluid temperatures as the best estimate of T.sub.d-1 or
T.sub.d-r. This estimated internal temperature of the piston or
dispensing chamber is then applied to Equation 2 above. In a
preferred embodiment, a look-up table of offsets may be constructed
and held in the memory of the system controller. As an alternative,
an estimate may be made for the particular dialysate system, or
type of system, as a generalized indicator of thermal performance.
An example is shown in FIG. 11. A temperature error is graphed on
the vertical axis and is well correlated with the average dialysate
fluid temperature (both in degrees C.) on the horizontal axis. In
one embodiment, the estimates of T.sub.d-1 and T.sub.d-r are
averaged and are correlated as shown in FIG. 11. The correlation is
stored in a look-up table in the memory of the system controller.
The temperature of the air was determined by a thermocouple placed
in the reference chamber. The error was determined by thermocouples
placed in the dialysate fluid itself to check the temperature
difference. In this series of experiments, the system included a
heated manifold and heated air tanks, as shown in FIG. 3, all at 57
C. The correlation was very high, R.sup.2 about 0.96.
[0056] As noted above, the temperature of the dialysate fluid
itself may be used, rather than relying on estimates from the
pneumatic system. One additional correction has also been
discovered that is based on the fluid temperature alone. In
calculating the volume of dialysate fluid, we have found that the
temperature of the fluid itself may cause an additional error. Our
results are shown in FIG. 12, which charts an error in fluid volume
against the temperature of the fluid itself. Our results show that
the error correlates with a particular correction, i.e., the true
volume delivered equals the indicated volume multiplied by a
correction factor. The correction factor is (-0.0014* fluid temp in
degrees C.)+1.0539. As an example, if the fluid temperature is 25
C, the correction factor is -0.0035+1.0539, for a total of 1.0504.
If the indicated volume is 1000 ml, the actual delivered volume was
1050 ml. Thus, if 1000 ml is meant for delivery, only 952 ml should
be delivered. This result was confirmed by experimenting with water
and measuring the mass of the volume.
[0057] The above disclosures are not meant to limit embodiments
discussed herein. For instance, referring to the temperature
control portion, it is clear that the manifold enclosure may be
integrated with the door assembly to achieve even greater thermal
stability and thus temperature control. This integration eliminates
error due to exposed tubing and could also lead to fewer heat
losses simply by increased proximity of the components. The above
discussions on thermal results and direct fluid measurement may
then be applied to the integrated fluid management system, with the
manifold enclosure with an integral door assembly experiencing
smaller thermal losses and error than at present. Look up tables as
discussed, for thermal lag, offsets, and so forth, may also be
constructed for the integrated assembly.
[0058] It should be understood that various changes and
modifications to the presently preferred embodiments described
herein will be apparent to those skilled in the art. Such changes
and modifications can be made without departing from the spirit and
scope of the present subject matter and without diminishing its
intended advantages. It is therefore intended that such changes and
modifications be covered by the appended claims.
* * * * *